There is a continuous need for reducing the memory requirements of a video decoder in order to reduce costs. However, it is also expected that the subjective quality of the decoded video will not deteriorate as a result. Memory reductions (for example in the ratio of 10:1 or higher) will also enable the effective embedding of memory components within a hardware decoder system component, similar to the embedded dynamic random access memory (“embedded DRAM”) technology.
For example, a typical video decoder chip like an MPEG-2 decoder uses a significant amount of memory for storing the data in frame buffers to be used for decoding temporally linked video frames, video post-processing and for storing on-screen display information for feature enhancements. The reduction of memory requirements, especially in relation to video decoding and post-processing, has been the subject of much research since it can provide significant savings in manufacturing costs.
Memory reduction with decimation in the spatial domain causes blurring of the image while decimation in the frequency domain by applying a fixed bit rate to encode a macroblock or block, as suggested in the prior art, causes unpredictable artefacts which are especially apparent in fast moving video sequences.
Another challenge of recompression is the precision of bit rate control. Since the size of the physical memory to be used in a system may be fixed, the rate control of the variable-length encoding circuit (or entropy encoding) must be accurate, such that the maximum memory is utilized without exceeding the allocated memory size. Known methods such as virtual-buffer-fullness control may not be used independently since the variations in the generation of bits would not be ideal for a fixed and maximally utilised memory buffer. A tighter control of accuracy for the virtual-buffer-fullness method results in the degradation of picture quality, while better picture quality is associated with a high variation in the bit rate.